The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting the presence of a substrate in a carrier head of a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. Therefore, the substrate surface is periodically planarized surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to affect the relative velocity distribution over the surface of the substrate. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
Typically, the carrier head is used to remove the substrate from the polishing pad after the polishing process has been completed. The substrate is vacuum-chucked to the underside of the carrier head. When the carrier head is retracted, the substrate is lifted off the polishing pad.
One problem that has been encountered in CMP is that the substrate may not be lifted by the carrier head. For example, if the surface tension binding the substrate to the polishing pad is greater than the force binding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. Also, if a defective substrate fractures during polishing, then the carrier head may be unable to remove the fractured substrate from the polishing pad.
A related problem is that the attachment of the substrate to the carrier head may fail, and the substrate may detach from the carrier head. This may occur if, for example, the substrate was attached to the carrier head by surface tension alone, rather than in combination with vacuum-chucking.
As such, an operator may not know that the carrier head no longer carries the substrate. The CMP apparatus will continue to operate even though the substrate is no longer present in the carrier head. This may decrease throughput. In addition, a loose substrate, i.e., one not attached to a carrier head, may be knocked about by the moving components of the CMP apparatus, potentially damaging the substrate or the polishing pad, or leaving debris which may damage other substrates.
Another problem encountered in CMP is the difficulty of determining whether the substrate is present in the carrier head. Because the substrate is located beneath the carrier head, it is difficult to determine by visual inspection whether the substrate is present in and properly attached to the carrier head. In addition, optical detection techniques are impeded by the presence of slurry.
A carrier head may include a rigid base having a bottom surface which serves as a substrate receiving surface. Multiple channels extend through the base to the substrate receiving surface. A pump or vacuum source can apply a vacuum to the channels. When air is pumped out of the channels, the substrate will be vacuum-chucked to the bottom surface of the base. A pressure sensor may be connected to a pressure line between the vacuum source and the channels in the carrier head. If the substrate was not successfully vacuum-chucked to the carrier head, then the channels will be open and air or other fluid will leak into the channels. On the other hand, if the substrate was successfully vacuum-chucked to the carrier head, then the channels will be sealed and air will not leak into the channels. Consequently, the pressure sensor will measure a higher vacuum or lower pressure when the substrate is successfully vacuum-chucked to the underside of the carrier head as compared to when the substrate is not properly attached to the carrier head.
Unfortunately, there are several problems with this method of detecting the presence of a substrate in the carrier head. Corrosive slurry may be suctioned into the channels and contaminate the carrier head. In addition, the threshold pressure for determining whether the substrate has been lifted from the polishing pad must be determined experimentally.
Accordingly, it would be useful to provide a CMP system capable of reliably sensing the presence of a substrate in a carrier head. It would also be useful if such a system could operate without exposing the interior of the carrier head to contamination by a slurry.
In one aspect, the invention is directed to a carrier head that has a base, a flexible member connected to the base defining a first chamber, and a support pad positioned between the base and the flexible member. A lower face of the flexible member provides a substrate receiving surface, and the support pad has a plurality of apertures which cooperate with the flexible member to provide a plurality of pockets to trap a fluid that may be located between a substrate and the flexible member as a result of a substrate attachment procedure.
Implementations of the invention may include the following features. A valve in the carrier head may open or close a passage in the carrier head. The plurality of apertures may include a sensor aperture positioned below the valve so that the flexible member can deflect into the sensor aperture to actuate the valve if a fluid is evacuated from the first chamber and the substrate is not attached to the substrate receiving surface. A support plate may be positioned between the support pad and the base, and the sensor aperture may be located below a corresponding aperture in the support plate. A guardian aperture may be positioned between an edge of the support pad and the sensor aperture to prevent the fluid from traveling directly from the edge of the an annular aperture substantially enclosing the sensor aperture. An annular wall may extend from the support pad towards the first flexible member about the sensor aperture. The wall may be substantially perpendicular to, or it may slope away from, a surface of the support pad. The plurality of apertures in the support pad may occupy a majority of a surface area of the support pad. The apertures may be of different sizes, or a uniform size. The carrier head may include a port fluidly coupled to the first chamber, and a pump may be coupled to the port to vary a pressure level in the first chamber. A support plate having a plurality of apertures may be positioned between the base and the support pad.
In another aspect, the invention is directed to a carrier head that has a base, a flexible member joined to the base to define a first chamber and a substrate receiving surface, and a support pad positioned between the base and the flexible member. The support pad includes a sensor aperture and a plurality of apertures extending about the sensor aperture to provide a plurality of pockets to trap a fluid which may be present between a substrate and the flexible member as a result of a substrate attachment procedure.
In another aspect, the invention is directed to a support pad for a carrier head that includes a flexible member which defines a substrate receiving surface. The support pad has a sensor aperture and a plurality of apertures extending about the sensor aperture to provide a plurality of pockets to trap a fluid which may be present between a substrate and the flexible member as a result of a substrate attachment procedure.
In another aspect, the invention is directed to a carrier head that has a base, a flexible member connected to the base to define a first chamber and a substrate receiving surface, a support pad positioned between the base and the flexible member, and a guardian member extending downward from the support pad. The support pad has a sensor aperture, and the guarding member forms a wall around the sensor aperture to block a fluid that can be trapped between the substrate and the flexible member during a substrate attachment procedure from entering the sensor aperture.
In another aspect, the invention is directed to a carrier head that has a base, a flexible member joined to the base to define a chamber and a substrate receiving surface, and a support plate positioned between the base and the flexible member. The support plate includes a sensor aperture and a plurality of apertures extending about the sensor aperture to provide a plurality of pockets to trap a fluid which may be present between a substrate and the flexible member as a result of a substrate attachment procedure.
In yet another aspect, the invention is directed to a carrier head that has a flexible membrane with a substrate receiving surface, a rigid body having an aperture into which the flexible membrane deflects if a chamber is evacuated and the substrate is not attached to the substrate receiving surface, and a barrier on the rigid body to prevent fluid that may be located between the substrate and the flexible membrane from causing the flexible membrane to deflect into the aperture when the substrate is attached to the substrate receiving surface.
Implementations of the invention may include the following features. The barrier may include a region into which the flexible membrane can expand to provide a fluid pocket. For example, the barrier may include a support pad having a plurality of apertures therethrough secured to a bottom surface of the body, or a plurality of recesses formed in a bottom surface of the body. The barrier may include a baffle joined to a bottom surface of the body.
In still another aspect, the invention is directed to a carrier head having a flexible membrane with a substrate receiving surface, a substrate detection mechanism, and means for preventing fluid that may be located between the substrate and the flexible membrane from interfering with the substrate detection mechanism.
In even another aspect, the invention is directed to a carrier head that has a base and a transparent flexible member connected to the base to provide a pressurizable chamber and a substrate receiving surface.
Implementations of the invention may include the following features. The flexible membrane may be formed of a silicone that does not contain pigmentation. A support structure may be positioned between the base and the flexible membrane, and a valve may be positioned on a lower surface of the base. The support structure may have an aperture therethrough which is aligned with the valve, and alignment of the valve with the aperture may be viewable through the transparent membrane.
Advantages of the invention include the following. The CMP apparatus includes a sensor to detect whether the substrate is properly attached to the carrier head. The sensor is less prone to false alarms.
Other advantages and features of the invention become apparent from the following description, including the drawings and claims.